The best way I find of completing Impossible the first time is to just use the Plasma Cutter. This limits you to only buying Plasma Energy and helps you to spend more money in Med Packs and nodes to upgrade your Rig and Stasis (which are crucial). In fact, I got the One Gun achievement and Epic 3 Tier Engineer on the same play through.

Sorry for the vague tip but if someone has already mentioned it then they know what they're talking about.